Assembly and method for wavelength calibration in an echelle spectrometer

ABSTRACT

A spectrometer assembly ( 10 ) is disclosed. The assembly includes a light source ( 11 ) with a continuous spectrum. A pre-monochromator ( 2 ) generates a spectrum with a relatively small linear dispersion from which a spectral portion is selectable, the spectral bandwidth of the spectral portion being smaller than or equal to the bandwidth of the free spectral range of the order in the echelle spectrum. The centre wavelength of the selected spectral interval is measurable with maximum blaze efficiency. The assembly also includes an echelle spectrometer ( 4 ) with means for wavelength calibration, an entrance slit ( 21 ) at the pre-monochromator ( 2 ), an intermediate slit assembly ( 50 ) with an intermediate slit ( 3 ) and a spatially resolving light detector ( 5 ) in the exit plane of the spectrometer for the detection of wavelength spectra.

This application claims the benefit of German Application No. 102 05142.9 filed Feb. 7, 2002 and PCT/EP03/00832 filed Jan. 28, 2003.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to a spectrometer. Furthermore, the inventionrelates to a method for the wavelength calibration of echellespectrometers.

Echelle spectrometers are spectrometers operating wi have a high angulardispersion, i.e. the ability to angularly separate closly proximatewavelengths. This has the advantage of a high resolution and yet smalldimensions of the assembly. Therefore, echelle gratings areparticularity suitable for high resolution spectroscopy, such as atomicabsorption spectroscopy with continuous light sources. A spectrometerwith an echelle grating normally operates in very high diffractionorders. Typical values are 20^(th) to 150^(th) order. The free spectralrange in each order is comparatively small.

In order to avoid spectral overlapping of different diffraction ordersin the exit plane of the spectrometer, echelle spectrometers are used incombination with an internal separation of the orders which isperpendicular to the direction of the echelle dispersion and leading totwo-dimensional spectra. The use of echelle gratings in combination witha pre-monochramator for the separation of the orders is also known as aso called double echelle spectrometer assembly. The radiation isspecrally pre-selected for example by means of a prism. Only theradiation from a limited spectral range essentially corresponding to oneorder enteres the echelle spectrometer. The generated echelle spectrumhas a linear spectral form. Diffraction gratings or prisms are used asdispersing optical elements for the selection of the order with apre-monochromator. The directions of the dispersion of thepre-monochromator and the echelle grating are parallel to each other.

For most applications it is necessary to calibrate the spectrometer. Awavelength is allocated to each geometric position in the exit plane ofthe spectrometer. The calibration can vary due to temperature changes,vibrations or other mechanical changes. In this case it may be necessaryto re-calibrate the device.

2. Prior Art

From the DE 41 18 760 A1 a double spectrometer assembly is known havinga fluid prism with variable prism angle which generates a spectrum witha low, adjustable dispersion. The spectrum is imaged on an intermediateslit simultaneously forming the entrance slit, i.e. the field stop forthe following echelle spectrometer. The intermediate slit cuts out apartial spectrum from the entire spectrum of the light source to bemeasured, the spectral band width of such partial spectrum being atleast smaller than the band width of the corresponding diffraction orderof the echelle grating. Such an assembly operates with a smallintermediate slit with constant width. The width of the intermediateslit is selected similar to the width of a picture element of thedetector (pixel). The width of the entrance slit is comparatively large.The selection of the spectral band width of the light entering throughthe intermediate slit is effected by varying the linear dispersion insuch a way that the prism angle is adjusted accordingly. The position ofthe wavelength of the portion of the spectrum is adjusted by rotatingthe prism. The position of the portion of the spectrum on the detectorof the echelle spectrometer is adjusted by rotating the echelle grating.The precision of the adjustment of the wavelength position is determinedby the precision of the mechanical adjustment of the angle of theechelle grating and the prism of the pre-monochromator, respectively.

From DE 195 45 178 A1, a spectrometer assembly is known consisting of anechelle spectrometer and a preceding prism spectrometer for theseparation of the orders, the assembly using the Neon spectrum of a lowpressure discharge lamp as a line source for the wavelength calibrationof the echelle spectrometer. The light from the line source enters theechelle spectrometer bypassing the prism spectrometer through anauxiliary slit in the plane of the intermediate slit and is detectedwith additional detector elements at the light detector. In such anassembly the widths of the auxiliary slit and the intermediate slit areconstant, have the same width and they are smaller than the entranceslit of the pre-monochromator. The intermediate slit forms the fieldstop for the double spectrometer assembly in a known way. The width ofthe entrance slit of the described assembly can be changed in steps.Using a fixed prism angle the entrance slit serves as the spectrallimitation of the light beam entering the echelle spectrometer. Thelight entering through the auxiliary slit without pre-dispersion for thewavelength calibration generates a characteristic pattern of spectrallines on the detector. Not all the lines belong to one diffraction orderof the Echelle grating, but represent the superposition of the differentdiffraction orders of the grating. Each line exactly represents one pairof values for the incident and diffraction angle at the grating. With asufficient line density at least on line is imaged on the referencedetector for each position of the grating. By mechanically coupling ofthe detectors on a common silicon chip a wavelength calibration of themeasuring detector can be performed for each measuring wavelength usingthe position of the reference line with the second, parallelly arrangedreference detector. The accuracy of the adjustment of the wavelengthposition is only determined by the measuring accuracy of the measurementof the reference spectrum, apart from the various imaging errors of themeasurement and reference spectra. Thereby, it is now independent fromthe accuracy of the mechanical adjustment of the Echelle grating.

It is a disadvantage of the known assemblies, that each detector elementof the light receiving detector is illuminated by the light from adifferent position of the entrance slit which is significantly widerthan the intermediate slit. Thereby a measuring error can be generated,when using the double spectrometer assembly especially for theinvestigation of light sources with an inhomogeneous light densitydistribution.

Furthermore, the accuracy of the adjustment of the wavelength positionof the wavelength range selected by means of the pre-monochromator iscompletely dominated by the accuracy of the mechanical adjustment of thedispersing element used for the pre-monochromator. Furthermore, the linedensity of the calibrating light source often is not sufficient withvery high linear dispersion of the echelle spectrometer to image atleast one calibration line on the detector for each grating position.

DISCLOSURE OF THE INVENTION

It is an object of the invention to provide a double spectrometerassembly of the above mentioned kind, where a complete wavelengthcalibration for the entire assembly is possible by controlled rotationof the echelle grating and the dispersing element of thepre-monochromator. Furthermore, it is an object of the invention toprovide a spectrometer, which can be calibrated, wherein the relativedistribution of the spectral intensity values within the selectedwavelength range is insensitive to intensity changes in the light sourcewith inhomogeneous light density distribution.

With the use of the narrow entrance slit and a wide intermediate slitthe entrance slit forms the field stop for the entire optical assembly.For different wavelengths, the same position of the light source isimaged on each detector pixel at all times. The image of the entranceslit can move due to, for example, thermal and mechanical influences.Due to the possibiltiy of imaging this image as a wavelength range of acontinuous spectrum on the detector and adjusting the pre-monochromatoralways in the same position on the detector this changing movement canbe compensated quasi-online. A highly precisely adjustable assembly isgenerated which provides correct spectral intensity values which areindependent of geometric changes in the light source and which are, to alarge extent, independent of thermal and mechanical influences. Thecontinuous spectrum ensures that a positionable intensity peak or apositionable intensity profile is present in each order and for eachgrating position. No ideal continuum is, however, required, in whichexactly the same intensity is present at all wavelengths. It issufficient if the light is emitted at all relevant wavelengths. Suchlight sources are, for example, noble gas high pressure short arc lamps.

The device is particularly suitable for applications using a lightsource with a continuous spectrum anyway. These are, amongst others,atomic absorption spectrometers, wherein the correction of backgroundinterference is effected with continuous light sources or atomicabsorption spectrometers with a continuous light source as a measuringlight source. However, the continuous light source can also be enteredinto the optical path exclusively for the calibration of the assembly.

The width of the entrance slit is preferably selected such, that thewidth of its image on the detector is equal to the width of a detectorelement. Thereby a good compromise between maximum resolution andmaximum light transmission is achieved. A reduction of the entrance slitwidth does not lead to an increase of the resolution. Enlargement wouldlead to higher light transmission, but, at the same time, would alsolead to decreasing resolution.

Preferably the width of the intermediate slit is adjustable. Then theintensity profile on the detector can be adjusted for each wavelengthsuch that only the required spectral bandwidth is used for spectroscopicmeasurements and all other wavelengths are blocked. This has, amongstothers, the advantage, that the stray light is reduced. For thecalibration of the pre-monochromator by means of a continuous spectrumthe measurement of the flancs of the intensity profile with minimum slitwidth can be used for the positioning.

In an embodiment of the invention the pre-monochromator comprises aprism. Quartz prisms in particular are very suitable for applications inthe UV/VIS-spectral range due to their high transmissivity. In a furtherembodiment of the invention the spatially resolving detector is a CCD-or a PDA-detector.

In a particularity preferred embodiment of the invention thepre-monochromator is arranged in a Littrow arrangement. Thereby acompact arrangement with small imaging errors and high resolution can beachieved with only few components. Spacial requirements and costs arethereby reduced. The same applies to the arrangement of the echellespectrometer.

Preferably the wavelength adjustment is effected by rotation of therespective dispersive elements. However, it is also possible to adjustthe other optical components, such as mirrors or the detector.

In a particularity preferred embodiment of the invention the means forthe wavelength calibration of the echelle spectrometer comprise a lightsource with a line spectrum, emitting light which can be imaged on theintermediate slit and means are provided for adjusting a line detectedwith the detector in a reference position. This kind of calibrationenables a wavelength adjustment in such a way that the remaining erroris determined by the measuring error of the detector only and not by theerror of the mechanical adjustment of the rotation of the grating. Theline spectrum of the calibration light source can occur in a wavelengthrange which is at a large distance from the wavelength to be measured,i.e. in a diffraction order, which is substantially different from theorder of the measuring wavelength, as long as the diffraction angles atthe echelle grating are sufficiently close to each other. By computingthe diffraction angle of the measuring wavelength and the referencewavelength in the different diffraction orders the corresponding angularposition of the grating can be adjusted in a very simple way.

In case that all overlapping lines of the calibration light source aretoo distant from each other for all diffraction orders of the echellegrating, in a further embodiment of the invention one or more additionalcalibrating slits can be provided next to the intermediate slit in thedirection of dispersion of the echelle grating and one ore more lightsources with line spectra for illuminating such calibrating slits can beprovided. In this case the line spectrum shifted in the direction ofdispersion occurs several times at the detector. Lines of the samewavelength are shifted by the geometric distance of their respectiveslit relative to the intermediate slit with respect to such line, whichis generated by the intermediate slit itself. The line density generatedthereby on the detector enables the reduction of the measuring errorwith the wavelength calibration. The real geometric distances betweenthe slit openings can be measured exactly by means of the detector forslit images of the same wavelength.

Preferably the prism and the echelle grating are arranged in such a waythat drifts of the image of the entrance slit in the intermediate slitand of the image of the entrance slit on the detector in the commondispersion direction of the prism pre-monochromator and echellespectrometer caused by changes of the prism- and grating dispersion dueto temperature changes occur in opposite directions. With increasingenvironmental temperature and the thermal extension of the gratingcarrier resulting therefrom the grating constant increases. Thereby thediffraction angle for a monochromatic wavelength decreases for aconstant incident angle at the echelle grating. The correspondingspectral line is shifted towards smaller wavelengths on the detector.

The environmental temperature also influences the diffraction constantof the prism material. Thereby the monochromatic image of the entranceslit in the intermediate slit is shifted. The echelle grating isilluminated with the incident angle of the wavelenght shifted in such away. A larger incident angle at the echelle grating results in a smallerdiffraction angle. With a suitable positioning of the grating, prismand—if present—optical components changing the dispersion direction(mirrors) both thermal effects can be made to operate in oppositedirections and, as a result, only cause a minimal drift of the spectrumon the detector. Thereby higher adjustment accuracy of thepre-positioning of the wavelength positions can be achieved. Furthermorethe required repetition rate of wavelength calibrations can be reduced.

The invention is described below in greater detail with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an echelle spectrometer withpre-monochromator.

FIG. 2 shows an intermediate slit assembly in detail.

FIG. 3 shows the intermediate slit assembly of FIG. 2 with additionalcalibration slits.

FIG. 4 a shows a portion with the intermediate slit and the calibratedposition of a monochromatic image of the entrance slit.

FIG. 4 b shows the intensity distribution of a light source withcontinuous spectrum and a light source with a line spectrum in areference position at the detector.

FIG. 5 a is a representation of FIG. 4 a in a non-calibrated position

FIG. 5 b is a representation of FIG. 5 b in a non-calibrated position

DESCRIPTION OF THE EMBODIMENT

In FIG. 1 numeral 10 denotes a spectrometer assembly. The assembly 10comprises a pre-monochromator 2 and an echelle spectrometer 4. Numeral21 denotes the entrance slit of the pre-monochromator 2. It isilluminated by a light source 11 with a continuous wavelength spectrumfor calibration. Such a light source is for example a Xenon highpressure short arc lamp. For this purpose a rotatable mirror 13 and alens 12 are arranged in the measuring optical path 14 as an imagingelement. The entrance slit 21 has a fixed slit width of 25 microns,corresponding to the width of a detector element 51 of a CCD-lineararray 5 used as a spatially resolving light detector. The entrance slit21 forms the field stop for the double spectrometer assembly and definesthe width of a monochromatic beam at the location of the detector 5.

The incident divergent light beam is deflected and collimated by aparaboloidal mirror 22. The parallel light passes through a prism 23.Thereby the light is dispersed for a first time. After the reflection ata plane mirror 24 which is arranged behind the prism 23, the radiationpasses through the prism 23 for a second time. Thereby almost a doublingof the dispersion is achieved. The light dispersed by the prism to adegree depending on the wavelength is then focused on the intermediateslit 3 by the mirror 22. Due to the double passage through the prism,maximum angular dispersion can be generated. This is particularlyimportant in the spectral range of long wavelengths over 600 nm to cutout sufficiently small spectral intervals from the continuous spectrumof the light source 11 at the intermediate slit 3. The positioning ofthe wavelength of the spectral range at the position of the intermediateslit 3 can be achieved by electromotoric rotation of the prism 23 andthe plane mirror 24 about a common axis 25. This is indicated by anarrow 26.

A portion of the light with a well-defined reduced spectral bandwidthenters the echelle spectrometer 4 through the intermediate slit 3.Therein, the divergent beam hits the paraboloidal mirror 41 and iscollimated. The paraboloidal mirror 41 reflects the parallel light beamin the direction of an echelle grating 42. After diffraction at theechelle grating 42 the light is again reflected by the paraboloidalmirror 41 and focused on the CCD-linear array 5. The latter converts thespectral intensity distribution to electric signals which afterwards aredigitised and transferred for further data processing. For the selectionof the wavelength, the echelle grating 42 can be rotated about arotational axis 43 extending parallel to the grooves of the grating.This is indicated by a double arrow 44.

The grating and the prism are mounted, in this assembly, in such a way,that the thermally caused wavelength drifts in the pre-monochromator andthe echelle spectrometer occur in opposite directions.

The intermediate slit 3 has two moveable slit jaws for adjusting thewidth of the intermediate slit. With the minimum adjustable width, theslit jaws touch a pin having a well defined diameter. Thereby, the slitjaws cannot approach each other any further and the slit width isadjusted to a reproducible value at a well-defined position. The minimumwidth is larger than the width of the detector elements and the entranceslit.

FIG. 2 shows an embodiment of the mechanical assembly 50 of theintermediate slit with adjustable slit width in detail. The intermediateslit is formed by two slit edges 52. The slit edges 52 are mounted on aflat slit edge carrier 57, respectively, which is connected to anessentially rectangular base body 58. The base body 58 is fixed to abase plate (not shown) of the spectrometer assembly. The connectingportion between the carrier 57 and the base body 58 is tapered and formsthe spring joint 51 of a bending spring.

The carriers 57 are cut out at the lower end in FIG. 2. An eccentricdisc 54 which is controlled by a stepper motor is arranged in the spaceformed thereby. If the eccentric disc 54 is rotated about an axis ofrotation 55, both carriers 57 are pushed apart against the force of thebending spring 51 or approach each other again. Therefore, the slitedges are not exactly parallel to each other, but perform ascissors-like movement. However, the influence on the limitation of thebandwidth generated thereby is negligible especially for small slitheights of, for example, 1 mm.

In order to provide sufficient space for movement of the carriers 57within the base body 58 spaces 59 are provided therebetween. Furthermorea pin 53 is provided defining the minimum slit width.

A further embodiment of the intermediate slit assembly 50 of the samekind is shown in FIG. 3. There, on each side of the base body 58, fixedauxiliary slits 56 are additionally provided. The auxiliary slits 56serve as additional calibration slits for increasing the line density atthe detector when the echelle spectrometer is calibrated.

Overall, the real image of the light source 11 in the entrance slit is,at first, exactly imaged on the intermediate slit 3 by the opticalsystem of the pre-monochromator 2 and, subsequently, it is imaged on thedetector by the optical system of the echelle spectrometer (FIG. 1).

By rotating a rotating mirror 33 into the measuring optical path thelight of a neon lamp can be focused by means of an imaging element 32 inthe plane of the intermediate slit 3, dispersed in the echellespectrometer, without preceding separation of the orders, and detectedas a spectrum on the detector 5.

The described assembly operates as follows for the wavelengthcalibration:

At first, the intermediate slit 3 is adjusted to a width which isslightly wider than the width of the entrance slit, for example to 30microns. Then the mirror 33 is rotated about the axis 34 into the lightpath. Thereby the light from the light source 11 with the continuousspectrum is blocked and the light from the light source 31 with a linespectrum is passed, through the intermediate slit, into the echellespectrometer.

The echelle grating 42 is roughly positioned by rotating about the axis43. This means, that a reference line selected for the calibration of adesired measuring wavelength can be unambiguously identified on thelinear detector array. This reference line is selected depending on themeasuring wavelength from a wavelength catalogue of known referencelines of the line source. The reached position of the reference line onthe linear detector array is determined. Then this position is comparedwith a previously calculated desired position. The desired position iscalculated from the difference between the calculated diffraction angleof the measuring wavelength and the reference line.

A deviation of the position of the reference line from its desiredposition is corrected by a fine correction of the angular position ofthe echelle grating and, thereby, also the measuring wavelength isadjusted to its desired position. This means that, by rotating thegrating, the line is shifted to its position. The echelle spectrometeris completely calibrated after this step. A wavelength can beunambiguously and very precisely allocated to each detector element,when light of a known order enters the spectrometer.

For the calibration of the prism arranged in the pre-monochromator theentrance slit is again illuminated by the continuum light source. Themirror 33 is again rotated out of the light path and the line spectrumis blocked. Here also the prism is first only roughly positioned. Thepositioning is effected in a way to ensure that the deviation from thedesired measuring wavelength is smaller than the section, detected bythe linear detector array, of the free spectral range of the order inthe echelle spectrum in which the measuring wavelength is measured withthe maximum blaze efficiency. In other words: The “correct” order iscoupled into the echelle spectrometer. In this case the spectral portioncan unambiguously be identified on the linear detector array.

As the intermediate slit is adjusted to a small width, the spectralinterval appears as a peak-shaped profile enabling the easydetermination of a maximum, a half-width value or the like. It is alsopossible to calibrate with a wider intermediate slit. In this case thespectral interval appears with a trapezoidal intensity profile, allowingthe definition of the position for example as the middle of thehalf-width value. The spectral interval is selected by the intermediateslit and dispersed at the echelle grating.

FIG. 4 a shows, at a heavily enlarged scale, the situation at theintermediate slit 3 of the pre-monochromator 2 in FIG. 1. The case ofthe ideal adjustment of the pre-monochromator is shown, after theposition of the echelle grating 42 (FIG. 1) has been exactly adjusted bymeans of the internal line source 31.

The measuring wavelength represented by the emission line 82 ispositioned exactly in the middle of the intermediate slit with the slitedges 80. The width of the intermediate slit determined by the distancebetween the slit edges 80 is selected such that, for the spectralbandwidth of the selected spectral interval its geometric width afterthe dispersion and imaging on the linear detector array 5 in the echellespectrometer 4 is smaller than the width of the detector. In the presentcase the width of the intermediate slit is between 0,05 and 0,1 mm. Thewidth of the linear detector array is about 10 mm.

FIG. 4 b shows the resulting intensity distributions on the lineardetector array 5 for the cases of the measurement of the emission line82 and a continuum 81 according to FIG. 4 a. The intensity distribution83 of the emission line 82 is symmetrical to its desired position 86after the calibration of the echelle spectrometer has been effected. Thecentre of the half-width value 85 of the essentially trapezoidalintensity distribution 84 for a spectral portion selected from acontinuum is exactly equal to the desired position 86 of the measuringwavelength after calibration of the pre-monochromator has beenperformed.

Due to temperature variations or other interference this situation canchange. Such a disturbed situation is shown in FIG. 5. FIG. 5 a shows,at a heavily enlarged scale, the intermediate slit in the case, wherethe spectrum of the pre-monochromator is shifted with respect to theintermediate slit. The measuring wavelength, represented by the shiftedemission line 92, is no longer located in the middle between the slitedges 80, i.e. also the position of the continuum 91 is shifted by thesame amount.

FIG. 5 b shows the associated intensity values on the linear detectorarray for the cases of measuring the emission line 92 and the continuum91. The centre position of the intensity distribution 93 of the emissionline 92 is shifted on the linear detector array by about the same amount98 from the desired position 86 as the emission line 92 is shifted fromthe centre between the slit edges 80 of the intermediate slit. However,the shift 99 of the centre of the half-width value 95 of the trapezoidalintensity distribution 94 of the spectral interval selected from acontinuum is considerably larger, because the spectral interval selectedby the intermediate slit, due to the echelle dispersion, appears to beheavily stretched on the linear detector array. This is the case becausethe echelle dispersion is considerably higher than the dispersion of thepre-monochromator. The image of a monochromatic emission line, however,is not broadened very much by the echelle dispersion.

A shift of such an intermediate image generates completely differentresults with respect to the intensity distribution on the lineardetector array, if the measuring spectrum is either a line spectrum or acontinuum. The much larger shift of the half-width value centre of thetrapezoidal intensity profile of the continuum as compared to the lineshift, which can be explained by the relation between the lineardispersions of the echelle spectrometer and the pre-monochromator,enables the highly accurate positioning of the measuring wavelength inthe intermediate slit using the continuum measurement.

The assembly is particularity suitable for measuring methods which makeuse of a light source emitting a continuous spectrum anyway, for exampleatomic absorption spectroscopy with continuum sources (CSAAS) or withatomic absorption spectroscopy with a background compensation by meansof a deuterium lamp.

1. A spectrometer assembly comprising a light source emitting light witha continuous wavelength spectrum, pre-monochromator means comprising anentrance slit, means for directing a light beam from said continuousspectrum light source onto said entrance slit of said pre-monochromatormeans, echelle spectrometer means comprising a blazed diffractiongrating with grooves and defining an exit plane, in which said echellespectrometer means in operation generates overlapping echelle spectra ofdifferent orders, each of said spectra having a free spectral range witha bandwidth, said blazed diffraction grating defining an order of anechelle spectrum, the center wavelength of which has a maximum blazeeffectiveness, said pre-monochromator means further comprising means forspectrally dispersing light from said light beam from said continuousspectrum light source with a degree of linear dispersion and means forselecting, from said dispersed light, a spectral interval having abandwidth, which is not larger than said bandwidth of said free spectralrange of said maximum effectiveness spectrum order and has an intensityversus wavelength profile, said profile defining a wavelengthcharacteristic of said profile, an intermediate slit assembly betweensaid pre-monochromator means and said echelle spectrometer means, saidintermediate slit means having a slit width and being arranged to forman exit slit of said pre-monochromator means and an entrance slit ofsaid echelle spectrometer means, said pre-monochromator being adapted togenerate a monochromatic image of said pre-monochromator entrance sliton said intermediate slit, said slit width of said intermediate slitassembly being larger than said monochromatic image of saidpre-monochromator entrance slit, and spatially resolving detector meanslocated in said exit plane for detecting said spectrum generated by saidechelle spectrometer means and defining a reference position thereon,calibrating means for calibrating said pre-monochromator to bring saidprofile characteristic wavelength to coincidence with said referenceposition.
 2. A spectrometer assembly as claimed in claim 1, wherein saidslit width of said intermediate slit assembly is adjustable.
 3. Aspectrometer assembly as claimed in claim 1, wherein said lightdispersing means of said pre-monochromator comprise a prism.
 4. Aspectrometer assembly as claimed in claim 1, wherein said spatiallyresolving detector is a CCD-detector.
 5. A spectrometer assembly asclaimed in claim 1, wherein said spatially resolving detector is aPDA-detector.
 6. A spectrometer assembly as claimed in claim 1, whereinsaid light dispersing means comprise a Littrow arrangement.
 7. Aspectrometer assembly as claimed in claim 1, wherein said blazeddiffraction grating of said echelle spectrometer is arranged in aLittrow arrangement.
 8. A spectrometer assembly as claimed in claim 1,and further comprising: means for calibrating said echelle spectrometer,said calibrating means comprising a light source emitting a linespectrum with a reference line, means for focusing the light from saidlight source on said intermediate slit, whereby said echelle spectrumgenerated on said spatially resolving detector represents said linespectrum, said spatially resolving detector defining an echellereference position thereon, and means for adjusting said echellespectrometer to bring said reference line in said echelle spectrum tocoincidence with said echelle reference position.
 9. A spectrometerassembly as claimed in claim 8, wherein said means for adjusting saidechelle spectrometer comprise means for rotating said grating about anaxis extending parallel to said grooves of said grating.
 10. Aspectrometer assembly as claimed in claim 1, and further comprising atleast one additional light source emitting a line spectrum with anadditional reference line, at least one additional calibration slit nextto said intermediate slit and associated with said at least oneadditional light source, means for focusing the light said at least oneadditional light source on said associated calibration slit, wherebysaid echelle spectrum generated on said spatially resolving detectorrepresents said line spectrum, said spatially resolving detectordefining an echelle reference position thereon, and means for adjustingsaid echelle spectrometer to bring said reference line in said echellespectrum to coincidence with said echelle reference position.
 11. Aspectrometer assembly as claimed in claim 1, wherein said lightdispersing means of said pre-monochromator means comprise a prism thedirection of dispersion of said prism being identical with the directionof dispersion of said echelle spectrometer means, the dispersion of saidprism being affected by temperature changes to cause a drift of saidmonochromatic image of said pre-monochromator entrance slit on saidintermediate slit, said echelle spectrometer generating, as said echellespectrum, a spectrally dispersed image of said pre-monochromatorentrance slit in said exit plane, said image being affected bytemperature changes to cause a drift of said image of saidpre-monochromator entrance slit in said exit plane, said prism and saidechelle grating being arranged to cause said drifts to substantiallycompensate each other.
 12. A method of calibrating a spectrometerassembly with pre-monochromator means having an entrance slit, echellespectrometer means, an intermediate slit therebetween, and a detector,comprising the steps of: calibrating said echelle spectrometer means,illuminating said entrance slit with light having a continuouswavelength spectrum, adjusting the width of said intermediate slit to avalue, with which a monochromatic real image of the entrance slit has asmaller width than the intermediate slit, detecting the position of thepassed spectral band at said detector and adjusting saidpre-monochromator means to position the passed spectral band at thedetector at a desired position.
 13. A method as claimed in claim 12,wherein said calibration of said echelle spectrometer means comprisesthe steps of: irradiating said intermediate slit with light from a lightsource having a line spectrum, determining the position of at least oneline of said line spectrum on said detector and comparing said positionwith at least one desired position, and adjusting said spectrometer tobring said at least one line on said detector into coincidence with saiddesired position.